The present invention relates to television picture distortion correction and more particularly to a scan compensation circuit for correcting raster position distortion, raster width distortion, and raster tearing distortion.
In modern television receivers it is standard practice to employ the horizontal output or flyback transformer of a horizontal sweep system as a source of high voltage bias for the cathode ray tube (CRT) of the receiver. During periods of high CRT brightness levels, instantaneous CRT beam current increases can subject the horizontal sweep system to loading resulting in several different forms of undesirable raster distortion.
In the case of a sharp transition from a low brightness area in the televised scene to an area of high brightness, instantaneous beam current sharply increases. If the area of high brightness is sufficiently extensive, such as in a scene including a sunset where a band of brightness extends across an otherwise predominantly dark scene, the duration of the instantaneous beam current rise will extend beyond the filtering time constant of the CRT anode capacitance. Although the longer time constant, automatic frequency control (AFC) circuit will respond eventually to compensate for certain effects of loading of the horizontal sweep system, during the time period before the AFC circuit responds, problems attendant to loading of the horizontal sweep system persist. This loading reduces the reactive energy level in the sweep transformer and in turn reduces the reactive energy level in the yoke. This process of energy level reduction causes a shift in horizontal sweep position which appears in the televised scene as position distortion.
High instantaneous beam current of duration longer than can be effectively filtered by CRT anode capacitance and of shorter duration than can be effectively dealt with by longer time constant circuitry such as AFC and beam current limiters will be referred to herein as short duration high beam current.
The loading of the horizontal sweep system also occasions lowering of the high voltage to the CRT so that electron energy decreases and the electron beams are subject to greater deflection than desired. Such unbalance results in size blooming or raster width distortion.
An attempted solution to correct raster width distortion is to deregulate the voltage supply to the horizontal output transformer to permit some tracking of horizontal yoke drive to CRT anode voltage. This can be occasioned by placing a resistor in series with the supply voltage so that the loading of the horizontal sweep system will result in a voltage drop across the series resistor to appear as a tracking drop in yoke drive. The compromise of such solutions is the reduced efficiency of the horizontal sweep system resulting in reduced image brightness and sharpness. Horizontal sweep system as used herein is intended to include the horizontal deflection and high voltage system. It is also intended that the words scan and sweep are synonymous.
Raster tearing distortion can occur when the short duration beam current is excessively high so that the horizontal sweep system is loaded to the point that the horizontal output transformer saturates. Where the duration of the excessive beam current is sufficient the beam current limiting circuit of the receiver, which senses essentially average beam current can effectively limit beam current. However, when the excessively high beam current rise occurs over a shorter period of time than can be responded to by the long time constant action of the beam current limiter, scan compensation correction is required.
The high brightness levels required by television users, particularly in viewing areas illuminated by fluorescent lighting such as is common in television showrooms, increases the need for a scan compensation circuit capable of controlling short duration high beam current related raster distortion.
It is accordingly an object of the present invention to control raster distortion due to short duration high beam current in the cathode ray tube of the television receiver.
It is another object of the present invention to control raster width, raster position, and raster tearing distortion due to short duration high beam current in the cathode ray tube of a television receiver.
Another object is to monitor horizontal sweep system loading and to utilize such loading as an input to control raster width, raster position and raster tearing distortion due to short duration high beam current in the cathode ray tube of a television receiver.
A solution to the above-noted raster distortions is to beef up the deflection system. Thus, with a more efficient yoke, a heftier flyback transformer and output transistor, the effects of system loading can be reduced. However, such a solution is the route of higher cost, higher power consumption and higher operating levels.
It is accordingly another object of the present invention to inexpensively correct raster position, raster width and raster tearing distortions by means of scan compensation.